November 05, 2011

The advantage of being first

"We find that families who are at the forefront of a range expansion into new territories had greater reproductive success. In other words, that they had more children, and more children who also had children," Labuda explained. "As a result, these families made a higher genetic contribution to the contemporary population than those who remained behind in what we call the range core, as opposed to the wave front.

The research confirms in humans a phenomenon that has already been observed in other species with much shorter generation spans. "We knew that the migration of species into new areas promoted the spread of rare mutations through a phenomenon known as 'gene surfing', but now we find that selection at the wave front could make this surfing much more efficient," Excoffier said. This evolutionary mechanism in combination with founder effects and social or cultural transmission of reproductive behavior could explain why some genetic diseases are found at an elevated frequency in the Charlevoix and Saguenay Lac Saint-Jean regions where the study was carried out, as rare mutations can also surf during a range expansion.

Science DOI: 10.1126/science.1212880

Deep Human Genealogies Reveal a Selective Advantage to Be on an Expanding Wave Front

Claudia Moreau et al.

Since their origin, human populations have colonized the whole planet, but the demographic processes governing range expansions are mostly unknown. We analyzed the genealogy of more than 1 million individuals resulting from a range expansion in Quebec between 1686 and 1960 and reconstructed the spatial dynamics of the expansion. We find that a majority of the present Saguenay Lac Saint-Jean population can be traced back to ancestors having lived directly on or close to the wave front. Ancestors located on the front contributed significantly more to the current gene pool than those from the range core, likely due to a 20% larger effective fertility of women on the wave front. This fitness component is heritable on the wave front and not in the core, implying that this life-history trait evolves during range expansions.

8 comments:

Well I'm surprised that nobody had reached to this conclusion before... It's not that difficult to imagine, if 30 couples arrive to one place, and then other 30 couples arrive 20 years later, the second wave is obviously not going to contribute 50% of the gene pool because the first wave has already had children.Also, the first to arrive are the ones who get better land, and take for themselves any resources available.

I'm surprised by the size of the research. One million people! That's a very big family tree.

It's not that difficult to imagine, if 30 couples arrive to one place, and then other 30 couples arrive 20 years later, the second wave is obviously not going to contribute 50% of the gene pool because the first wave has already had children.

I don't think that's it, at all. 20% more children is highly significant, and their children had more, as well.

In this particular case, new settlers needed to have above-average resources to finance the equipment and/or livestock, grains, and transport in the first place, and needed to have above average education/knowledge to both know about the opportunity and to be successful.

More importantly, they likely also came from big families, since if they had few siblings, there wouldn't have been much pressure to leave above-average living circumstances. Finally, they clearly were risk-takers who on average have children earlier.

Conversely, later waves were usually less-educated also-runs, usually with little resources, not necessarily coming from large, successful families, and with much lower success rates.

"More importantly, they likely also came from big families, since if they had few siblings, there wouldn't have been much pressure to leave above-average living circumstances. Finally, they clearly were risk-takers who on average have children earlier".

I think the solution is even simpler. The first arrivals, whether human or other, have access to almost unlimited resources. Unlimited resources promote rapid expansion of population numbers. Such a scenario appears to have happened with the arrival of Polynesians in New Zealand, although the evidence is not widely publicised. The population expanded rapidly until the resources ran out. War and starvation reduced the population to the numbers when Europeans first arrived. There is evidence that the population had been almost five times as large as that within 300 years of Polynesian's arrival.

"As a result, these families made a higher genetic contribution to the contemporary population than those who remained behind in what we call the range core, as opposed to the wave front".

which is the main reason I have come to believe that Y-hap C and mtDNA N arrived in Australia before Y-hap K and mtDNA M did.

"Conversely, fertility is a fickle item, ranging from zero to 15+ offspring everything else held equal".

Fertility will only really be a problem if the incoming group is particularly inbred.

"Thing is, resources in technologically newly-covered areas are plenty for many generations".

And I have to correct a comment I made concerning first arrivals. Groups invading an area already occupied can come to dominate the haplogroups already there if the incomers have a 'superior' technology. I'm reasonably certain that is the case with Y-hap O in SE Asia. The hill country in South China and SE Asia was reasonably sparsely settled before the early Neolithic began expanding from the Yangtze basin.

This would be easy to replicate in another group. It would also help differentiate between a lot of these hypothesis that people have proposed. If you look at Utah for example, you have some early pioneers who arrived in the 1840 and 1850s and then missionary work continued, mainly in the British Isles and Scandanavia, these settlers continued to arrive for generations until church leaders recommended that people stay in their home lands and build up the church there rather than emigrate. Utah has a giant genealogical database that spans its entire history. There would be some confounding variables though because some (actually a small percentage) early pioneers practiced Polygamy until 1890. This would increase the fertility rate of the men and decrease the fertility rate of the women. But definitely, it would be very interesting to study.

You could also look at siblings, ones who did join the church and came to Utah and compare them to their siblings who did not join the church and remained in their native lands. I can almost guarantee that you would see increased fertility in the Utah "expansion" group.

Fertility will only really be a problem if the incoming group is particularly inbred.

Terry,

I have no idea what you mean by that. Infertility is roughly as high in couples with vastly divergent backgrounds as it is in those with somewhat close proximity. The (short-term) "optimum" is second cousins, according to some studies. Few couples not practicing contraception have more than 6-8 live children. But some have 15 - 20.

Clearly, there is a huge range of factors operable, here, and if you come from a family with many children, and an economically successful one to boot, chance is that you will have above-average fertility - no question.

"Few couples not practicing contraception have more than 6-8 live children. But some have 15 - 20".

Population growth is largely a product of offspring survival rather than straight 'fertility' anyway. And that survival is a product of availability of resources. Most populations have had no trouble reaching maximum carrying capacity within just a few generations. That applies to all species, not just humans. Starting with just one couple and allowing a generous 25 years per generation, even at an average of just 10 children per couple the population would grow exponentially to reach three quarters of a million within two hundred years. With a larger founder population growth would be even more rapid. That fact alone ensures that there is a huge advantage in being first. The only factor that would limit such exponential growth is if the original population was extremely inbred.

"The (short-term) 'optimum' is second cousins, according to some studies".

Yes. Very much short-term optimum. With any sort of continued inbreeding fertility drops away. Inbreeding has possibly been responsible for the extinction of human populations on several isolated islands, for example.

"Infertility is roughly as high in couples with vastly divergent backgrounds as it is in those with somewhat close proximity".

In animal breeding it has been noticed time and again that with inbreeding the first thing that falls off is fertility, especially survivability of offspring. Presumably the same would be true of humans.